The long term objectives of the proposed research are to improve our understanding of the immune response to Mycobacterium leprae and other bacterial infections, to produce more specific and powerful reagents for serodiagnosis and for assessing delayed type hypersensitivity in leprosy, and to develop an inexpensive live mycobacterial vaccine that provides lasting protective immunity against M. leprae. To accomplish these goals, the specific aims of the experiments outlined in this proposal are: 1) To identify and isolate M. leprae genes that specify immunologically relevant proteins by probing a Lambdagt11 recombinant DNA expression library with monoclonal and polyclonal antibodies that bind M. leprae antigens. 2) To isolate T cell clones from leprosy patients, contacts, and immunized individuals and to use them to identify individual protein antigens and epitopes, expressed by recombinant DNA clones, that may be important in immunological protection or suppression. 3) To test individual M. leprae antigens produced in E. coli for their utility in leprosy serodiagnosis and delayed hypersensitivity. 4) To develop a transformation method that allows stable introduction of foreign DNA into BCG (a vaccine strain used for tuberculosis immunoprophylaxis) and/or M. smegmatis (a possible vaccine strain). 5) To stably transform BCG and/or M. smegmatis with DNA vectors containing genes for antigens of M. leprae that can be expressed in their new host and are potentially important for protective immunity. 6) To test the recombinant mycobacteria for their ability to induce antibody formation, delayed type hypersensitivity and protection against disease in animal models. The health relatedness of this project lies in the development of diagnostic and vaccine reagents for over a billion individuals at risk of contracting leprosy and in the project's potential contributions to understanding the immunological basis of resistance to M. leprae and other intracellular pathogens.